1. Field of the Invention
The invention relates to an ultrasonic flowmeter for measuring the flow of a flowing medium having a measuring tube and having an ultrasonic transducer, wherein the measuring tube has a transducer pocket, wherein the ultrasonic transducer is provided having contact to the flowing medium in the transducer pocket of the measuring tube and has a transducer housing and a transducer element and wherein the transducer housing has an ultrasound window.
2. Description of Related Art
The ultrasonic flowmeter described above is a normal ultrasonic flowmeter insofar as it has a measuring tube and an ultrasonic transducer. In the following, however, an ultrasonic flowmeter should also be understood as one that does not have a separate measuring tube, but instead the measuring tube is an integral part of a pipe, for example, a piping system having a medium, whose flow is to be measured, flowing through it.
The use of ultrasonic flowmeters has gained increasingly in importance over time in operational measurement of flow of liquids and gases, summarized as flowing media. Flow measurement with the help of ultrasonic flowmeters is carried out as in, for example, magnetic-inductive flowmeters “without contact”, i.e., without interfering installations, which result in vortices and an increased loss in pressure.
In ultrasonic flowmeters, the method of measurement is differentiated primarily between the Doppler method and the running time method, and in the running time method, there are the direct running time method, the pulse repetition frequency method and the phase shift method (see H. Berard “Ultraschall-Durchflussmessung” in “Sensoren, Messaufnehmer”, published by Bonfig/Bartz/Wolff in expert verlag, as well as the VDI/VDE guideline 2642 “Ultraschall-Durchflussmessung von Flüssigkeiten in voll durchströmten Rohrleitungen”).
A measuring tube, which generally represents the measuring section, having an inlet section and an outlet section, on the one hand, and on the other hand, at least one ultrasonic transducer, which is sometimes called measuring head, are essential for operation of ultrasonic flowmeters of the type being discussed. An ultrasonic transducer is to be understood in a general sense here. Ultrasonic transmitters, i.e., measuring heads for generating and emitting ultrasonic signals, on the one hand, and on the other hand, ultrasonic receivers, i.e., measuring heads for receiving ultrasonic signals and for converting the received ultrasonic signals into electric signals, are both parts of the ultrasonic transducers. However, measuring heads that combine an ultrasonic transmitter and an ultrasonic receiver in one, i.e., are used both for generating and transmitting ultrasonic signals as well as receiving ultrasonic signals and converting the received ultrasonic signals into electric signals, are also ultrasonic transducers.
An ultrasonic transducer of the type discussed last is installed in ultrasonic flowmeters that operate with only one ultrasonic transducer. Such ultrasonic flowmeters determine the velocity of the flowing medium with the help of the Doppler shift of an ultrasonic signal reflected on an inhomogeneity of the flowing medium. Likewise, it is possible that the Doppler shift of the ultrasonic signals can be determined via two ultrasonic transducers arranged without being offset on opposing sides of the measuring tube.
It is also possible to perform ultrasonic flow measurements that are based on the running time method and in which two ultrasonic transducers are used, arranged offset on the same side of the measuring tube in the direction of flow, wherein the ultrasonic signals are reflected on the side of the measuring tube opposite the ultrasonic transducers. However, two ultrasonic transducers are regularly provided that are arranged offset, opposite one another in the direction of flow.
It is stated above that a measuring tube and an ultrasonic transducer are parts of an ultrasonic flowmeter of the type being discussed here, that the measuring tube has a transducer pocket, and that the ultrasonic transducer is provided in contact with the flowing medium in the transducer pocket of the measuring tube. The invention relates, of course, also to an ultrasonic flowmeter having several ultrasonic transducers, in which the measuring tube consequently has several transducer pockets.
Transducer pocket in the scope of the invention means a recess or cavity, implemented in any sort of manner, located outside of the flow cross section of the measuring tube, in which the ultrasonic transducer is installed so that it does not extend into the flow cross section, or at least does not extend substantially into the flow cross section, i.e., does not influence or does not essentially influence the flow. When several ultrasonic transducers are provided offset and opposite one another in the direction of flow, then they are aligned in respect to one another. Generally, the longitudinal axis of the transducer pockets runs at an acute angle or an obtuse angle to the direction of flow of the flowing medium or to the longitudinal axis of the measuring tube (see image 6.1.1, page 532 of the citation “Sensoren, Messaufnehmer” l.c., image 8, page 18 of the VDI/VDE guideline 2642 l.c. and FIG. 2-2 on page 21 of the citation “Ultrasonic Measurements for Process Control” from Lawrence C. Lynnworth, ACADE-MIC PRESS, INC., published by Harcourt Brace Jovanovich).
There are also ultrasonic flowmeters in which the ultrasonic transducers do not come into contact with the flowing medium; i.e., are arranged on the outside of the measuring tube, so-called “clamp-on arrangement”. The invention relates, however, only to ultrasonic flowmeters in which the ultrasonic transducers come into contact with the flowing medium.
The flow of the medium flowing in the measuring tube does not remain without influence by the transducer pocket, in fact vortices are generated in the flow by the transducer pockets. The examination of the formation and the description of the formation process of vortices is a current field of scientific research. Essentially, the forming of vortices can be described using the cavity resonance theory. This theory is summarized in the following.
Without limiting universality, the explanations are described using a cavity located on the measuring tube in the form of an open cube, wherein the open side faces toward the flow channel. These explanations, however, are applicable for all forms of transducer pockets. The cavity has five closed sides, namely a cavity floor and four side surfaces, wherein only two side surfaces, namely a first side surface perpendicular to the direction of flow and a second side surface perpendicular to the direction of flow, are relevant for the explanations, wherein the first side surface is located before the second side surface in respect to the direction of flow.
Phenomena or mechanisms that dominate in the forming of vortices are, on the one hand, so-called shear layer modes, and on the other hand, so-called wake modes. A free shear layer is described in general as the transition area between two parallel flows with different velocities. Shear layer modes are dependent of the length and the depth of the cavity, the Mach number, wherein the Mach number describes the ratio of the velocity of the medium to the speed of sound in the medium M=U∞/c, and the interface layer thickness δ. The flow region close to the measuring tube wall is the interface layer, in which the forces of viscosity and the forces of inertia are in the same order of magnitude. Here, the section influenced by the forces of viscosity is designated as the interface layer thickness. The forming of the vortex caused by shear layer modes is described as follows:
A free shear layer is formed between the cavity and the “outer area” located in the measuring tube. This free shear layer is generally instable and has interferences. The interferences of the free shear layer encompassing the cavity strike the second side surface of the cavity, i.e., the back, side surface in respect to direction of flow. An acoustic pulse or an acoustic wave is generated by the stagnating pressure, which spreads upstream. This acoustic wave causes a pressure difference between the acoustic wave spreading below the shear layer and the acoustic wave continuously spreading above the shear layer. This pressure difference influences the shear layer in such a manner that further interferences occur in the shear layer, the shear layer “rolls up” and a vortex is formed that spreads downstream. This vortex moving downstream strikes, in turn, the second side surface of the cavity. Thus, the feedback chain for exciting and maintaining the system is closed and further vortices are formed. A flow state dominated by periodic unsteady pressure fluctuations is thus formed.
The cavity resonance frequency for rectangular cavities can be determined using an empirical formula developed by Rossiter. It is
      St    n    =                              f          n                ⁢        L                    U        ∞              =                  n        -        α                    M        +                  1          /          κ                    
Here, Stn is the Strouhal number, fn is the vortex shedding frequency, L is the length of the cavity, U∞ is the velocity of flow, α is a factor that gives the temporal delay between the occurrence of a shear layer interference and the emission or kindling of an acoustic wave/an acoustic pulse at the second side surface of the cavity, κ is the ratio between the vortex convection velocity and the free flow velocity of the medium and M is the Mach number.
It can be seen using the described formula that the Strouhal number Stn is dependent on the Mach number M. However, it has been seen in experiments that there are also cases in which the Strouhal number Stn is quasi constant, thus nearly independent of the Mach number M. This leads to the assumption that the mechanism for forming a vortex based on shear layers modes cannot be the only one, but that a further mechanism has to exist that is based on purely hydrodynamic instability phenomena. This further mechanism is described by so-called wake modes. Wake modes occur mainly at high Reynold's numbers. The vortices formed by the mechanism have a lower shedding frequency than vortices caused by shear layer modes. Wake modes are characterized by strong unsteady flow with chaotic behavior caused by the interaction being clearly more pronounced between the free shear layer and the cavity flow.
As a solution for the problem described above, that results from vortices generated from transducer pockets, it has already been suggested to fill the transducer pocket with plastic (see, FIG. 4-9 on page 257 of the citation “Ultrasonic Measurements for Process Control”, l.c.). However, the same disadvantages arise as in ultrasonic flowmeters based on the Snell's law, in which the ultrasonic transducer is attached to the outside of the measuring tube, i.e., in a so-called “clamp-on arrangement”. Additionally, there are problems with the acoustic impedance and problems with the plastic filled into the transducer pockets, in particular at high temperatures. The disadvantages and problems associated with filling the transducer pockets with plastic are why this embodiment was not established in practice.
Professionals in the field have already dealt with the problems resulting from the vortices generated due to transducer pockets. Here, reference is made to German Patent DE 196 48 784 C2 which corresponds to U.S. Pat. Nos. 6,189,389 B1, 6,748,811 B1 and International Patent application Publication WO 2012 084392 A1. Ultrasonic flowmeters are known from German Patent DE 196 48 784 C2, U.S. Pat. Nos. 6,189,389 B1 and 6,748,811 B1, in which the transducer pocket is provided with a mesh grating. Another embodiment is shown in International Patent application Publication WO 2012 084392 A1, namely one in which a baffle plate is inserted perpendicular to the ultrasound window of the ultrasonic transducer in front of the ultrasound window of the ultrasonic transducer in the transducer pocket of the measuring tube.